Life is short--and that's particularly true for animals that mature fast and die young. A new study reveals that some of these creatures are in such a hurry to get on with things that they prefer a fever over a full-fledged immune response to deal with infections. The same species also appear less likely to interrupt their daily life to recover from illness.
Animals launch direct assaults against bacterial and viral invaders with their immune system. But they have other strategies to cope with infection, such as raising body temperature and adopting "sickness behavior"--that is, decreasing food intake, becoming lethargic, and reducing activities such as sex and aggression. Each of these mechanisms carries a cost: Forgoing sex reduces fitness, for instance, and heightening body temperature by just 1 degree Celsius can mean using 10% more energy, says evolutionary ecologist Lynn Martin of the University of South Florida in Tampa.
So far, little research has been done on which choices various species make, let alone why, but Martin and his colleagues suspected that the decisions might depend on life strategy. So they took five closely related mouse species from Canada, the United States, and Mexico and injected between four and nine individuals of each with lipopolysaccharide, a bacterial component known to simulate infection. The researchers then monitored the animals' body temperature and activity pattern for 24 hours.
The team found that white-footed mice (Peromyscus leucopus) and deer mice (P. maniculatus)--two species that live fast--rapidly developed a high fever. But Aztec mice (P. Aztec) and plateau mice (P. melanophrys)--both slower-living species--had little or no fever at all. A third slow-living species, the cactus mouse, (P. californicus), actually lowered its body temperature, the team reports in Functional Ecology.
The explanation for the difference may be that fever "is sort of throwing a hand grenade into the system," Martin says. It not only helps clear pathogens but also causes collateral damage to the body's own tissues that might shorten life span. That would make fever suitable for fast-living organisms, which wouldn't care about long-term damage, whereas slower-living species would prefer to rely on antibodies, a more targeted strategy. But the simple fact that fever is fast may also be important for fast-living organisms, says Martin: "If your life is short, you don't want to spend 3 or 4 weeks recovering" from an infection. Sheer time constraints may also explain why the fast-living mice showed little or no sickness behavior in the study, whereas the slow-living ones did.
Many people had speculated that different life strategies, even in closely related species, might have led to different types of defenses against pathogens, says Kirk Klasing, an animal scientist at the University of California, Davis. "But this is the first major piece of evidence," he says.